Friday, December 28, 2012

As name suggests ClassCastExceptionin Java
comes when we try to type cast an object and object is not of the type we are
trying to cast into. In fact ClassCastException in Java is
one of most common exception in Java along with java.lang.OutOfMemoryError
and ClassNotFoundException
in Java before Generics was introduced in Java 5 to avoid frequent
instances of java.lang.classcastexception cannot be cast towhile
working with Collection classes like ArrayList
and HashMap,
which were not type-safe before Java 5. Though we can minimize and avoid java.lang.ClassCastExceptionin Java by
using Generics and writing type-safe
parameterized classes and method, its good to know real cause of ClassCastExceptionand How to
solve it. In this Java tutorial we will see Why java.lang.ClassCastException
comes and how to resolve ClassCastExceptionin Java. We
will also see how to minimize or avoid ClassCastExceptionin Java by
using Generics,
as prevention is always better than cure.

Cause of java.lang.ClassCastException in
Java

In order to understand cause of ClassCastException, you need
to be familiar with concept of type
casting in Java. Since Java is an object oriented programming language and
supports features like Inheritance and Polymorphism, a reference variable of
type parent class can represent object of child class. This leads to ClassCastException
if object is not of type on which you are casting it. It can be best
explained with an example. Let's see a sample code which will throw ClassCastExceptionin Java:

If you see closely we were trying to cast an String object
into an Integer object which is not correct and
that’s why Java throws java.lang.classcastexception cannot be cast to error. Problems can become more
difficult if all class are closely related. Since due to polymorphism
in Java an Object instance can hold any type of Object but you can only
cast between same type. what makes this problem worse is that it only comes
during runtime, ClassCastException doesn't come at compile time
which makes it hard to detect specially in large enterprise Java application or
high frequency electronic trading system where input comes from upstream and in
production various kinds of input are available. This was frequent problem with
Java Collection classes like LinkedList
and HashSet
in Java which holds Object type but with introduction of Generics
in Java 5 solved this problem by checking type-safety during compile time. Which
means you can not store Integers on LinkedList of String, from Java 5 it will
result in compile time error.

Common
source of java.lang.ClassCastException

Following are some of the most common source of ClassCastExcepiton in
Java:

1. Java Collection classes like HashMap, ArrayList, Vector
or Hashtable which is not using Generics.

2. Methods which were written to take advantage of polymorphic behavior
and coded on interfaces prior to Java 5 and doesn’t used parameter to provide
type-safety. look for all places where you have used cast operator in Java and verify
that whether that code is type-safe or not.

Here are some of the most frequently occurred ClassCastException in Java:

This will come when you try to cast String object to Integer i.e.Integer number = (Integer) stringObject;

java.lang.classcastexception java.lang.string cannot be cast to java.util.date :This error will come when you cast String to Date in Java, since both are not related to each other, it will not possible to typecast them.

java.lang.classcastexception java.util.arraylist cannot be cast to java.lang.string
I have seen this happening a lot in beginners code based on Java Collection framework, Since ArrayList and String are not related to each other, following casting will throw this error :

java.lang.classcastexception java.util.arraylist cannot be cast to java.util.mapjava.lang.classcastexception ljava.lang.object cannot be cast to ljava.lang.comparablejava.lang.classcastexception ljava.lang.object cannot be cast to ljava.lang.integerjava.lang.classcastexception ljava.lang.object cannot be cast to java.util.listjava.util.arraylist cannot be cast to java.lang.comparable

How to fix java.lang.ClassCastException in Java

Solving ClassCastException can be very easy once you
understand polymorphism in Java and difference
between compile time and runtime things. ClassCastException are simple
like NullPointerException
just look the stack-trace and go to the line number. Many of advanced Java IDE
like Eclipse and Netbeans will give you hyperlink to
navigate till culprit line number in Java file. Now you know where exactly ClassCastException
is coming and stack trace also told which object it was trying to cast,
Now you have to find, how that type of object comes and this could take some time
based upon complexity of your java application. You can also prevent ClassCastExceptionin Java by
using Generics. Generics are designed to write type-safe code and provides
compile time checks which tends to violate type-safety. By using Generics in Collection
classes and other places you can safely minimize java.lang.ClassCastException
in Java.

That’s all on What is ClassCastException in Java and how to solve Exception in thread "main" java.lang.ClassCastException. I highly
recommend to use Generics while using any Collection classes or class which
acts as container e.g. ThreadLocal
in Java. Also write your own classes and method to take advantage of
Generics to provide type-safety as shown in this example of How
to write parameterized class. It’s also a good idea to refresh your
knowledge on type-casting
in Java.